NON-PARASITIC, OR PHYSIOLOGIC PLANT DISEASES 567 



trees may be prevented by proper shelter, or by liberal mulching. 

 Sometimes a light straw shelter, or wind-break, may be efficacious. 



Water-logging. — Transpiration from the leaves of plants is much 

 reduced during periods of long-continued rains or fogs and as a result the 

 plant becomes gorged with water. Growth is stimulated, but the cells 

 are thin walled and easily dry up, or are the easy prey of fungi and in- 

 sects. Such excess of water may result in the formation of little warts 

 and swellings. These may be formed on leaves or stems. Sometimes 

 the leaves become diseased by being water-logged in spots which are 

 translucent in appearance. Galloway and Woods^^ describe the in- 

 fluence of the excess of water during the season of 1896 in Washington, 

 D. C. " In early spring vegetation was at first a little retarded by cool 

 weather, but this was suddenly followed by good growing weather, 

 during which the leaves of most trees and shrubs especially those of 

 Norway maples pushed out with great rapidity. This latter period was 

 followed by one quite dry and warm, during which red spiders increased 

 to unusual numbers, particularly on the lower and more protected leaves 

 of the crown. After this came a period of several days of rainy weather, 

 and many of the spiders were washed off, but the leaves where they had 

 been working became water-logged. The Norway maples and horse- 

 chestnuts suffered most, the leaves of these trees in many cases appear- 

 ing to have been scorched with fire." 



Such injuries as water-logging resulting from an excess of moisture 

 in the air cannot be prevented readily. Proper planting may render 

 trees less liable to such trouble especially if care is exercised in feeding 

 them after they are planted. Susceptible trees such as horse-chestnut 

 and Norway maple require special care and if the conditions under 

 which these trees can be grown open the way to serious water-logging 

 they should be discarded and other trees planted in their stead. 



Qidema of Manihot. — The blister-like pustular outgrowths on plants 

 variously designated as oedemata or intumescences have been the subject 

 of careful investigation by a number of plant pathologists. The disease 

 is also known as dropsy^ and has been observed both in greenhouses 

 and out-of-doors (Fig. 206). The diseased condition known as oedema 

 or dropsy occurs on stems, leaves and fruits. It has been found recently 



^ Galloway, B. T. and Woods, Albert F.: Diseases of Shade and Ornamental 

 Trees. Yearbook, U. S. Dept. Agric, 1896: 245. 



^ SoRAUER, Paul, Lindau, G. and Reh, L.: Manual of Plant Diseases, trans. 

 by Frances Dorrance, i: 335. 



